In general, a particle beam therapeutic device is used to irradiate the affected part of a cancer patient with a particle beam (hereinafter referred to merely as “beam”) such as a carbon beam or proton beam. A particle beam irradiation method currently used includes an expanded beam method. In the expanded beam method, a beam diameter of the particle beam is expanded to a size equal to or larger than the size of the affected part of a cancer patient. However, the expanded beam method cannot strictly three-dimensionally match the beam with the shape of the affected part, and there is a limit in reducing influence on normal tissues around the affected part.
Then, as a further advanced irradiation method of particle beam therapy, a scanning irradiation method is now being put into practice. This method virtually divides the affected part of a patient into a three-dimensional lattice points and performs irradiation to each lattice point. This scanning irradiation method includes, for example, a three-dimensional irradiation method called “spot scanning irradiation method”. In the spot scanning irradiation method, each spot (point) is irradiated in the following way.
When a predetermined dose of radiation has been applied to a certain spot in an affected part under the control of a beam emission control device, the scanning control device outputs a spot switching command signal upon receiving a radiation dosage termination signal from a dose monitor. Based on the spot switching command signal, the beam emission control device stops beam emission.
At the same time, an electromagnet power supply for supplying an exciting current to an irradiation field forming electromagnet to scan the particle beam starts setting a current value corresponding to the coordinates of the next irradiation spot. The scanning control device, upon receiving a setting completion signal for the set current value of the electromagnet power supply, outputs a beam irradiation start command signal to the beam emission control device, and the irradiation to the next point is started. This is sequentially repeated to irradiate a treatment area with respect to one irradiation slice.
When the irradiation for one irradiation slice is completed, the beam emission is temporarily interrupted. Then, the beam termination position (slice) in the beam travelling direction changed by changing energy of the beam emitted from an accelerator or by controlling a range adjustment apparatus called a range shifter. In this way, the scanning irradiation and the slice switching are sequentially performed for irradiation over the entire treatment area.
In the above scanning irradiation method, a position monitor is provided at an irradiation port to allow the operator to check if the beam is being irradiated to a correct position. If there occurs a current setting abnormality of the electromagnet power supply for supplying an exciting current to the irradiation field forming electromagnet, or trajectory deviation of beam in the beam transport direction from an upstream-side accelerator to a downstream side scanning irradiator, a difference occurs between an irradiation trajectory pre-determined and an irradiation trajectory measured by the position monitor. In this case, an interlock signal (emergency stop signal) is output from a position monitor controller provided in the scanning control device to interrupt therapeutic irradiation.
Possible causes for the beam trajectory deviation include, for example, a change in the magnetic field of an electromagnet in a path for transporting the beam from an accelerator to a treatment room. When such a magnetic field change occurs, the beam is not transported on a correct trajectory, making it impossible to ensure quality sufficient as the therapeutic beam.
To operate a particle beam therapeutic device that employs the scanning irradiation method, every morning an operator corrects setting values set in an operation unit for operating the beam for each irradiation beam set energy while checking beam trajectory to thereby check the quality of the beam used for scheduled therapy.
Specifically, a pair of screen monitors each forming a fluorescent film are disposed on the beam trajectory so as to be spaced apart from each other at a predetermined distance. The operator adjusts the current values of correcting electromagnets based on a deviation amount (deviation amount from an ideal center trajectory free from beam trajectory deviation) calculated from output values from these screen monitors, thereby correcting the beam trajectory.
Further, even when the setting value set in the operation unit for operating the beam remains unchanged, it may not be always true, because of a temperature change or the like, that a deviation amount of the beam trajectory during the morning and that during the afternoon are the same. Thus, when a beam position abnormality is detected by the position monitor at the irradiation start or during irradiation, the therapeutic irradiation may not be accomplished.
In the technique described in Patent Document 1, there are provided a first beam position monitor that detects a beam passing position on the irradiation nozzle upstream side and a second beam position monitor that detects a beam passing position on the irradiation nozzle downstream side.